1. Field of the Invention
The invention relates to a system for the acquisition of data measured in a well or borehole and more particularly seismic data.
The acquisition of measurement data in a borehole is generally achieved by lowering therein one or more probes suspended from a multifunction cable. The probe has an elongate body and one or more mobile arms.
Hydraulic jacks move each arm from a rest position against the body of the probe to an open position in which they are anchored in the wall of the borehole and thus immobilize the probe. Hydraulic circuits connect the jacks to means for delivering pressurized fluid generally comprising an electric motor and electrovalves.
2. Description of the Prior Art
The multi-function cable very often comprises a plurality of conducting lines for transmitting electric currents for supplying the motor and for signals of any kind. It is a question for example of control signals transmitted to the probe from a laboratory at surface as well as response signals sent from the probe. The response signals are formed either by monitoring or test signals, or data measured by the sensors of the probe. Such a probe is described for example in U.S. patent 4,428,422.
A probe may for example also be used having anchorage arms actuated by springs held under tension by bolts during lowering. Hydraulic means comprising electrovalves are used for unlocking and opening the arms.
The electrovalves are actuated by control signals received through the multi-function cable. Such a probe is described for example in U.S. patent 4,616,703.
The different sensors lowered into the well for making measurements of different kinds may be included in a single probe or else in a set of probes comprising a main probe under which a plurality of satellite probes are suspended at different depths. Such an assembly of probes is described for example in French patent FR-2 564 599.
Seismic prospection operations are for example carried out by disposing in wells a plurality of sensors adapted for detecting waves reflected by the discontinuities of the subsoil, coming from a seismic source disposed on the surface or in the vicinity thereof or else in another well. The signals received are used so as to obtain seismic cross sections of the subsoil.
The seismic sensors are generally directional and oriented in one or more directions perpendicular to each other. They are disposed in an assembly of several probes so as to pick up the seismic signals received at different depth levels and are connected to a surface laboratory, either directly by a special line included in the multi-function cable, or by means of a seismic acquisition apparatus;
In this latter case, the different signals are collected, sampled, digitized and transmitted through one or more lines of the multi-function cable in coded form. After decoding, they are recorded and processed by a seismic laboratory.
Increasing the number of sensors and probes required for the seismic prospection in boreholes, makes the lowering operations, positioning and checking of the correct operation of the apparatus more and more complex. This is all the more so that operations are carried out at considerable depth under harsher temperature and pressure conditions.
A conventional method for monitoring the lowering of a single probe in a well consists in checking the tension of the support cable. A stop in the advance causes tension discontinuities which are readily detectable. When a reception device is used comprising several probes, this detection method is practically no longer possible for the variations of tension caused by stopping the single satellite probe at the head of the advance are much smaller. In the case of late detection, the borehole risks being jammed which is difficult to resolve.